Conformal coating is the process of applying a dielectric material onto a substrate. Typically, the substrate is an electrical product, such as a printed circuit (“PC”) board or a device mounted thereon. Conformal coating, also called film coating protects the electrical components on the PC board from moisture, fungus, dust, corrosion, abrasion and other environmental stresses. Common conformal coating materials include silicone, acrylic, polyurethane, epoxy, synthetic resins and various polymers. When applied to PC boards, an insulative resin film of uniform thickness is formed as a solvent evaporates or, as a solvent free material is cured. Current applications require the conformal coating to be applied onto selected areas of the PC board and over some or all of the components thereon in order to preserve electrical and/or heat conduction properties. Applicators are conventionally pneumatically or electrically actuated, for example. In the case of pneumatically actuated applicators, an actuation valve, such as a solenoid valve, is used to supply positively pressurized actuation air to a piston chamber in the applicator in order to move a valve stem in the applicator to an open position. While the valve of the applicator is open, the film of coating material will be dispensed onto the substrate.
Automated systems may have one or several conformal coating applicators mounted on a robotic system. Machine speeds have gradually become faster and, therefore, faster actuation valves are used to cycle the applicator on and off as the robotic system moves the applicator relative to the PC board to selectively apply conformal coating to components on the board. The valve stem of the actuator reciprocates between a closed position in which a first end is engaged against valve seat, and an open position in which a second end is engaged against a hard stop element. Typically, the stop element is part of a stroke adjuster that allows the valve stem travel distance, or stroke, between the open and closed positions to be changed according to the application needs. With faster actuation valves, it has become more common to experience a “rebounding” effect as the valve stem impacts against the stop element at the end of the opening stroke. That is, the top end or second end of the valve stem will impact against the hard stop element, such as a stroke adjustment screw, and rebound or move slightly in the opposite direction one or more times before coming to a complete stop against the stop element. This rebounding movement will cause disruptions in the flow pattern at the leading end of the pattern (e.g., film) being applied to the substrate. For example, this phenomenon can cause the leading end of the film coating strip to have an undesirable “hammer head” or slightly wider shape than the remaining portions of the film strip.
It would therefore be desirable to provide a conformal coating applicator and method that prevent or at least reduce disruptions in the liquid flow upon opening the valve associated with the applicator.